Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PARTICULATE INJECTION BURNER
The present invention relates to a burner for injecting, such as particulate
material,
material and relates particularly, but not exclusively, to such a burner for
use in an
electric arc furnace.
It is well known to provide an electric arc furnace with supplementary oxygen
injection lances; operation of such a furnace involves the striking of an arc
between
electrodes which creates a heating current which passes through the metal to
be
melted and the injection of supplementary oxygen via the oxygen injection
lances,
which may be moved closer to or away from the metal as and when desired. Once
struck, the arc acts to heat the metal towards its final temperature of about
1620 C
to 1700 C whilst the oxygen acts to oxidise undesirable elements in the metal
and
causes them to be extracted from the metal and generate an insulating slag
layer
which floats on the surface of the molten metal. The insulating slag layer
acts to
protect the electrodes and furnace wall from splattering molten metal.
Supplementary oxy/fuel burners are often provided in the furnace wall for
assisting
the electric arc heating effect. Our European patent application number
0764815 A
describes an oxy/fuel burner intended to reduce the problem whereby such
burners
are unable to penetrate the slag layer adequately during the final and
critical heating
step in conventional electric arc furnaces.
A further problem with conventional electric arc furnaces occurs when it is
necessary
to introduce particulate material into the furnace in order to assist in the
thermal and/or
chemical processes occurring therein. It is difficult to ensure that such
particulate
material is correctly distributed and/or delivered to the correct region of
the furnace.
It is an object of the present invention to reduce and possibly eliminate the
above-mentioned problems associated with the introduction of particulate
material into
furnaces, such as electric arc furnaces.
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Accordingly, the present invention provides a burner for use in an electric-
arc furnace
comprising a body portion having a longitudinal axis X and a main outlet
located
thereon, fuel and primary oxidant outlets upstream of said main outlet and
disposed
substantially concentrically about axis X, a chamber within the body portion
for
receiving and mixing said fuel and oxidant and acceleration means downstream
of
said chamber for causing said mixture of fuel and oxidant to be accelerated
towards
and out of said main outlet for combustion, wherein means are provided for
discharging particulate matter entrained in a secondary oxidant into the flow
of
accelerated fuel and primary oxidant immediately adjacent and downstream of
said
accelerating means.
With such an arrangement the oxidant-entrained particulate matter is drawn
into the
accelerating flow of fuel and primary oxidant to be thoroughly distributed
and/or to
reach the desired location within the furnace. Where the particulate matter is
coal,
partial or even total devolatilisation can be achieved in the flame, the
volatiles
providing further fuel for combustion and hence providing fuel savings.
The means for accelerating the flow of fuel and primary oxidant preferably
comprises
a flow path for the mixture which successively converges and diverges in the
direction of flow.
The accelerating means may comprise a Laval nozzle substantially coaxial with
axis X, the discharging means being disposed substantially concentrically
about
axis X. Preferably the discharging means are configured so as to discharge the
oxidant-entrained particulate matter substantially parallel to the axis X.
The discharging means may conveniently be in the form of an annulus
surrounding
the accelerating means, being adapted to discharge the oxidant-entrained
particulate matter in a hollow, substantially cylindrical or conical, spray
pattern. With
such an arrangement, the discharge means may be configured so as to provide a
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linear flow path for the particulate matter (ie a flow path which is
substantially parallel
along the significant portion of its length) which is particularly suitable
when the
particulate material is one with significant abrasive qualities, such as iron
carbide.
Alternatively, the discharge means may be substantially coaxial with the axis
X, the
accelerating means being concentrically disposed around the discharge means.
The accelerating means may suitably have an outlet in the form of an annular
surrounding the discharge means.
In such an arrangement, the acceleration of the fuel and primary oxidant from
an
annular outlet produces a significant pressure reduction adjacent the
discharge
means and therefore provides enhanced mixing and penetration of the
particulate
material. The discharge means may also be shaped and configured so as to
accelerate the oxidant-entrained particulate matter discharged therefrom,
thereby
accelerating the particulate material to an even greater extent.
The present invention also affords a method of operation of a burner for an
electric
arc furnace, the method comprising accelerating a mixture of fuel and primary
oxidant towards and out of a main outlet of a burner body for combustion, and
discharging particulate matter entrained in a secondary oxidant adjacent to
accelerating flow of fuel and primary oxidant, whereby said oxidant-entrained
particulate matter is drawn into the flow of fuel and primary oxidant.
In most electric arc furnace applications the fuel would be natural gas. The
primary
oxidant may be oxygen or oxygen enriched air and the secondary oxidant for
entraining the particulate material is preferably air, although it could be
identical to
the primary oxidant in some applications. Moreover, although the present
invention
is described above in relation to the injection of particulate material, we
have
discovered that certain embodiments of burners in accordance with this
invention
are particularly suitable for the injection of liquids (such as additional
liquid fuel or
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cryogenic liquids such as liquid oxygen, as may be desirable in certain
applications)
or for the injection of slurries (ie particulate materials entrained in a
liquid), as in the
drying and/or incineration of waste sludge, such as sewage. In either case,
the
liquid material is entrained in air, as with the injection of particulate
material, but in
droplet or atomised form. Accordingly where used herein, and particularly in
the
Claims, the term "particulate material" should be understood to encompass both
discrete droplets of liquid and of particulate material entrained in liquid.
Embodiments in accordance with the invention will now be described by way of
example and with reference to the accompanying drawings, in which:
Figure 1 is a cross sectional view of part of the outlet end of a burner in
accordance
with a first embodiment of the invention, and
Figure 2 is a cross sectional view of the outlet end of a second embodiment of
a
burner in accordance with the invention;
Figure 3 is a cross sectional view of a third embodiment of a burner in
accordance
with the invention, and
Figures 4a to 4d are cross sectional views of the various elements of the
burner of
Figure 3.
Figure 1 shows, in schematic cross section, the outlet end of a burner 1 (for
clarity
only part of the burner 1 is shown in Figure 1; it should be understood that
the
burner of Figure 1 is substantially symmetrical about longitudinal axis X).
Burner 1 comprises a "rocket burner" nozzle, of the type well known in the
art,
shown generally at 3. Nozzle 3 emits natural gas and oxygen, with an oxidant
to
fuel mol ratio of less than or equal to 2:1, into housing 5. In the direction
of ffow (to
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the right in Figure 1) the flow passage for the mixture of fuel gas and oxygen
is
radiused at 7, 9 and 11 so as to form a "Laval nozzle", that is a successively
convergent and divergent flow path which serves to accelerate the flow of fuel
and
primary oxidant, and also to enhance mixing thereof. Surrounding housing 5 is
a
further, outer, housing 13 which defines an annular flow path, or passage, 15
between housing 5 and the inner portion of outer housing 13. Flow passage 15
is
provided for the introduction of particulate material into the flow of fuel
and primary
oxidant. The particular material, which is entrained in air, flows along flow
path 15,
from left to right in the diagram, until, in the region adjacent the distal
end 17 of
housing 5 the pressure drop created by the acceleration of the flow of fuel
and
oxidant therepast draws in the flow of air entrained particulate material,
mixing it with
the flow of fuel and hence propelling it with the burner flame away from the
distal
end 19 of burner 1, thereby ensuring that the particulate material is fully
distributed
within the flame produced by burner 1 and is projected as far as possible into
the
electric arc furnace (not shown).
A significant feature of the burner 1 of Figure 1 is that flow path 15 is
straight (ie
there are no curves or obstructions therein). This is important for avoiding
erosion of
parts of the burner 1 by the particulate material where that material is of a
particularly abrasive nature (such as in the case of iron carbide).
The inner housing 5 is preferably water cooled at its distal end (as shown
generally
by reference 21), and the outer housing 13 is provided with a flow path 23 for
cooling purposes (for a fiow of cooling water or air).
As will be apparent to those skilled in the art that the air entraining the
particulate
material flowing from flow path 15 provides a valuable source of secondary
oxidant
for the combustion process, thereby providing a staged flame which, as is
known in
the art, helps reduce harmful NOX emissions.
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The burner 51 shown in Figure 2 comprises an outer housing 53 and an inner
housing 55 which together provide a successively convergent and divergent flow
path 57 in the form of an annulus for the fuel (natural gas) and the oxygen,
or
oxygen-enriched air supplied via annular channels 59, 61 respectively. The
convergent/divergent flow path 57 serves to accelerate the flow of fuel and
oxidant
to be discharged from the main outlet 63 of burner 51 for subsequent
combustion.
The housings 53, 55 (which are water cooled) are radiused, respectively, at
65a, 65b
and 65c, 65d so as to create the successively convergent and divergent flow
path 57
from left to right in Figure 2.
Inner housing 55 also defines a convergent flow path 67 for a supply of
particulate
material, such as coal, entrained in air, which flow of particulate material
is drawn by
the reduction in pressure created by the annular flow of accelerating fuel and
oxidant
mixture emitted from flow path 57 so as to mix thoroughly therewith as the
combined
flow moves away from the distal end 63 of burner 51. The annulus of
accelerating
flow of fuel and mixture produced by the burner of Figure 2 produces a
significant
drawing effect on the particulate material fed along flow path 67, promoting
thorough
mixing and projection of the particulate material. This is particularly
suitable for
introducing a particulate fuel material into the flame.
In the burner 51 shown in Figure 2, when operated as a coal/air and
natural gas/oxygen burner/lance, with an oxygen supply along outlet 61 of
about
35 psi or more (about 0.24 MPa or more) with a natural gas supply of greater
than
4 MW, and a pressure of about 25 psi or more (about 0.17 MPa or more) a
maximum flow rate of greater than 50 kilograms per minute of particulate coal
is
possible.
Those skilled in the art will appreciate that the burner of Figure 2 is
particularly
suitable for introducing a flame into an electric arc furnace at sonic or
supersonic
speeds but that the particulate flow in flow path 67 may lead to unacceptable
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abrasion of the inner housing 55 (particularly in the regions shown by
references 65c
and 65d), particularly where the particulate material is abrasive. Thus,
although
suited for use with pulverised or particulate coal, the burner 51 of Figure 2
may
suffer unacceptable abrasion when used with harder particulate materials, such
as
pulverised coke or particulate char (partially oxidised coal) or iron carbide;
the burner
shown in Figure 1 is more suited for use with these types of particulate
materials.
The burner 101 shown in Figure 3 is very similar to the embodiment of Figure 2
except that the central, particulate flow path 103 has no curves or
restrictions
therein, which is particularly desirable when injecting large volumes of
particulate
material, or particularly abrasive material, or when injecting droplets of
liquid or
slurries of particulate material in a liquid.
Primary oxidant such as oxygen and gaseous fuel such as natural gas are
directed,
via inlets 105 and 107 respectively, to mix in convergent/divergent flow path
107,
which is in the form of an annulus centred on axis X. Particulate material
entrained
in secondary oxidant passing along flow path 103 is entrained in the
accelerated
flow emitted from flow path 109, the particulate material being fully
distributed
throughout the combustion zone.
The distribution of particulate matter throughout the flame is advantageous as
it
preheats the particulate material before it enters the furnace. Where the
particulate
material is coal, preheating can partially or even totally devolatilise the
coal particles,
the released volatiles serving as fuel for combustion and the remainder
consisting
mainly of carbon.
The burner 101 of Figure 3 is provided with water inlets 111, 113 and
corresponding
water outlets 117, 115 for a flow of water to cool the burner in use.
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Figures 4a and 4b show the burner of Figure 3 partly disassembled and figures
4c
and 4d show the sub-assembly of Figure 4b disassembled. As can be seen, the
largely axial-symmetric construction illustrated in Figure 3 allows for quick
and easy
assembly and disassembly of burner 101, for maintenance and repair or for
exchange so as to accommodate different types or flow rates of fuel, oxidant
and/or
particulate matter.
Although principally described in relation to the injection of particulate
coal into an
electric arc furnace, burners in accordance with the present invention can be
used in
many other applications (the injection of non-reactive solid material, such as
the
preheating of waste dust for reintroduction into an electric arc furnace, for
example),
and with liquids or slurries, in droplet or atomised form. Burners in
accordance with
the invention are not restricted to use in electric arc furnaces, but can also
be used
in incineration, drying and various iron and steelmaking processes, in cupola
furnaces, DRI and iron carbide production.
By supersonic injection of hot oxygen (superstoichiometric flame) it is
possible to
use the burner for decarburisation of the metal as well as post combustion (of
carbon monoxide). The burner can be mounted in a water-cooled box. This box
can
be fitted with an oxygen port or lance for introducing extra oxygen for post
combustion while the burner injects hot oxygen and carbon for slag foaming.
As is known to those skilled in the art, the different parts of the burners
shown in
Figures 1, 2 and 3 are configured and dimensioned to take account of such
variables as the backpressures available, particle size and desired flow rate,
flow
rates/velocities to be achieved and the calorific output required from the
burner. It
will also be understood that the burner of the present invention is not
limited to any
particular fuel/oxidant ratio; in certain applications it is desirable to
provide an
oxidant-rich fuel/oxygen mixture ("superstoichiometric running"), such as in
post
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combustion processes, or slag foaming, whereas in other applications- it is
desirable
to provide an oxidant-poor ("substoichiometric") mixture.
